Tools
Histology, Natural Killer Cells
Introduction
Natural killer or NK cells are a subpopulation of large granular lymphocytes that arise from a common NK/T-cell progenitor. Accumulating evidence indicates that NK cells can develop and mature both in the bone marrow and secondary lymphoid tissues including tonsils, spleen, and lymph nodes.[1] In the innate immune system, NK cells participate in early cytotoxic responses against viruses, parasites, microbial pathogens, as well as tumor immunosurveillance. Also, their protective and pathogenic roles have been more defined in the development of autoimmune diseases such as systemic lupus erythematosus, type I diabetes mellitus, and autoimmune liver disease.[2] NK cell's function is regulated by various activating or inhibitory receptors. These germ-line encoded immunoreceptors on the surface of NK cells sense nonspecific alterations of self-cells caused by cellular stress, infections, or malignant transformation.[3] In contrast, B and T lymphocytes in the adaptive arm, undergo gene rearrangement to generate surface receptors and mediate the antigen-specific immune responses.[4][5]
While encountering normal cells, inhibitory receptors override activating receptors. However, molecular changes on the cell surface due to viral infection or tumor formation can stimulate the activating receptors. Upon activation, NK cells release granzyme, perforin, effector molecules of the tumor necrosis factor family, and Fas-ligand to induce apoptosis of the target cells. In addition, NK cells synthesize and release other cytokines and chemokines such as Il-10, gamma-interferon, and granulocyte-macrophage colony-stimulating factor, to recruit other immune effector cells to the target site. NK cells also exhibit antibody-dependent cell cytotoxicity against cells or target antigens which are marked with specific antibodies. Recently, therapeutic applications modulating the inhibition-activation dynamic of NK cells have been developed for multiple conditions including cancer, autoimmune disease, and diabetes.[6]
Issues of Concern
Register For Free And Read The Full Article
Search engine and full access to all medical articles- 10 free questions in your specialty
- Free CME/CE Activities
- Free daily question in your email
- Save favorite articles to your dashboard
- Emails offering discounts
Learn more about a Subscription to StatPearls Point-of-Care
Issues of Concern
Natural Killer cells represent the third lymphocyte population (Following T and B cells), and they play essential roles in the frontline's defense of the innate immune system. The term natural killer suggests that these cells can kill the target cells without pre-stimulation. In contrast to other immune cells, which demand a considerable period to induce their cytolytic activity, NK cells provide ready-to-kill machinery in the immune system. These cells play various roles in several organ systems; for instance, there is emerging evidence linking NK cells with the development of autism spectrum disorder. The lower degrees of NK cell infiltration in solid tumors correlates with a favorable prognosis. In addition, excessive stimulation of NK cells promotes cell senescence in tissues, laying the foundations for aging. NK cells are important for the body's salutogenic response in dealing with various physiological and non-physiological stimuli throughout life.
Structure
Morphologically, NK cells are large, granular lymphocytes that are defined by expression of CD56 and CD16 and lack the common T cell marker, CD3, and T cell receptor in humans. They constitute 5% to 15% of the total population of peripheral mononuclear circulating lymphocytes.[7] NK cells, dendritic cells, B, and T lymphocytes are derived from single lymphoid progenitor cells. A variety of activating or inhibitory receptors have been demonstrated on the surface of NK cells as their immunoreceptor. Inhibitory receptors have cytoplasmic tyrosine-based inhibition motifs and recognize self-major histocompatibility complex class I (MHC I) molecules expressed by most of the nucleated cells. However, activating receptors recognize the ligands exhibited by pathological cells and interact with cell signaling pathways to initiate effector activities. Examples of their activating receptors are CD16, NKp30, NKp44, NKp46, NKG2C/CD94, and NKG2D/CD94. NK cells have distinct cytoplasmic granules that are structurally and functionally similar to lysosomes, hence called “secretory lysosomes.” These granules have a bi-layered membrane that separates their lytic content from the cytoplasm.[8]
Function
Natural killer cells' effector functions are tightly regulated by the balance of inhibitory and activating receptors and downstream intracellular signaling cascades of these receptors. NK cells were originally identified in the 1970s,[9][10] although the mechanism by which they distinguish healthy cells from virus-infected or tumor cells was not fully known for years. First, the results of research on murine NK cells demonstrated that NK cells can detect and eliminate lymphoma cells that lost the expression of MHC I, whereas MHC I-expressing tumor cells are unaffected by NK cells[3]. This led to the idea of missing self in the activation of NK cells, particularly in their anti-tumor activity. In physiological conditions, most of the circulating NK cells are resting as their inhibitory receptors interact with MHC I in the healthy cells, and this accounts for exhibiting self-tolerance. Almost every healthy cell expresses MHC I molecules, a potential ligand for inhibitory NK cell receptors, However, MHC I molecules are downregulated upon viral infection and tumor transformation (missing self), exerting subthreshold inhibitory signaling to the NK cells, moving their balance toward the activated state.
In addition, upregulation of NK cells activating receptors occurs in stressed or damaged cells resulting in targeting these cells for the cytolytic activity of NK cells. Interaction between CD16 on NK cells with antigens expressed on tumor cells causes antibody-dependent cell-mediated cytotoxicity tumor cell lysis by NK cells.[11][12][13][14][15] NK cells can kill tumor cells without previous sensitization. They directly kill tumor cells by releasing lytic granules that contain granzyme and perforin. NK cells, together with cytotoxic CD8+ T lymphocytes, generate immunity against virus-infected and tumor cells. In most circumstances, tumor cells downregulate their MHC I to escape from cytotoxic T cells. However, the lower expression of MHC I makes tumor cells susceptible to lysis by NK cells.[16]
Histochemistry and Cytochemistry
NK cells are considered the equivalent of CD8-positive T lymphocytes in the innate immune system and multiple cell markers are used for their characterization in immunocytochemistry staining. Natural killer cells are usually classified as CD3 negative and CD56 positive cells and subdivided into CD56dim and CD56bright groups. CD56dim and CD56bright subsets are defined based on the density of CD56 and have distinct functional attributes. About 90% of circulating and spleen natural killer cells are CD56dim, CD16 positive, and express perforin. CD56dim cells mainly perform a cytotoxic action and CD16 mediates ADCC. In comparison, NK cells in lymph nodes and peripheral lymphoid organs (eg, tonsils) are CD56bright and CD16 negative. CD56 bright cells are mainly cytokine-secreting cells and can produce several cytokines, including IFN-γ (gamma-interferon) upon stimulation.[16][17][18]
Microscopy, Light
Natural killer cells present a distinct group of circulating lymphocytes on the peripheral blood smear. On Wright-Giemsa-staining, they are visible as large granular lymphocyte morphology. They possess large nuclei containing coarse chromatin and prominent nuclei. Their abundant basophilic cytoplasm contains plenty of azurophilic granules that are negative for peroxidase staining.[19]
Microscopy, Electron
NK cells are large and exhibit various shapes and express microvilli more prominently in the area of effector-to-target contact. At the same time, microvilli are scant cell surfaces in the absence of target cells. Membranes are not extensively fused in the region of target and effector cell contact. NK cells have structurally distinct granules that consist of two different compartments. The outer compartment includes the lysosome-associated phosphatase acid enzymes and trimetaphosphatase. No enzymatic activities associated with the inner compartment have been identified. At the time of degranulation, a limited area of the cytoplasm exhibits vacuole-like areas possessing granules and apparent granular debris. Degranulation appears to be implicated in NK cells' cytotoxic activity. Convoluted nuclei with distinctive polarity against the dense granules and pseudopodia are typical. In the cytoplasm, both Mitochondria and polysomes are observed.[19][20]
Pathophysiology
In addition to NK cell roles in defense against viral infections and tumor immunosurveillance, they have been linked to several other diseases including asthma [21], inflammatory bowel disease [22], and autoimmune diseases [23]. Advancements in understanding the pathophysiology of NK cells resulted in the development of new therapeutic options and treatments. For instance, NK cells are found in the pancreas of patients with diabetes type I, and they enhance the disease development. Reducing the NK cell activation using specific blocking antibodies will prevent further disease progression.[6]
Clinical Significance
NK cells are essential components of innate immunity. Although their function may be compromised in many primary immune deficiency disorders, rare isolated Natural Killer cell deficiency syndromes may also occur. There are two potential varieties of NK cell deficiency: (1) the classical natural killer cell deficiency (CNKD) and (2) functional natural killer cell deficiency (FNKD).[24][25] CNKD reflects the significant absence of NK cells among peripheral blood mononuclear cells and fewer than 1% of NK cells in peripheral blood should be present in a minimum of three independent tests separated by one month to establish this diagnosis. Unlike CNKD, patients with FNKD have NK cells found in their peripheral blood within the spectrum of age-specific average values; however, permanently deficient in their functions. Both CNKD and FNKD patients are highly susceptible to human papillomavirus, herpes simplex virus, cytomegalovirus, Epstein-Barr virus, varicella-zoster virus infection.[24][26][27]
The prognostic value of NK cells infiltrating solid tumors has attracted considerable attention in clinical practice. Several independent studies have demonstrated that higher levels of NK cell markers such as CD56 and NKp46 in solid tumors correlate with favorable prognosis and overall survival rate.[28][29][30] However, further research is necessary to establish a scoring system including information from tumor type, clinical staging, pathological grading, etc. The clinical development of NK cell-based cancer therapy provided promising results. The NK cell-based clinical trials presented a desirable safety profile and efficacy in hematologic malignancies and solid tumors.[31][32][33]
References
Scoville SD, Freud AG, Caligiuri MA. Modeling Human Natural Killer Cell Development in the Era of Innate Lymphoid Cells. Frontiers in immunology. 2017:8():360. doi: 10.3389/fimmu.2017.00360. Epub 2017 Mar 27 [PubMed PMID: 28396671]
Liu M, Liang S, Zhang C. NK Cells in Autoimmune Diseases: Protective or Pathogenic? Frontiers in immunology. 2021:12():624687. doi: 10.3389/fimmu.2021.624687. Epub 2021 Mar 12 [PubMed PMID: 33777006]
Sivori S, Vacca P, Del Zotto G, Munari E, Mingari MC, Moretta L. Human NK cells: surface receptors, inhibitory checkpoints, and translational applications. Cellular & molecular immunology. 2019 May:16(5):430-441. doi: 10.1038/s41423-019-0206-4. Epub 2019 Feb 18 [PubMed PMID: 30778167]
Carrillo-Bustamante P,Keşmir C,de Boer RJ, The evolution of natural killer cell receptors. Immunogenetics. 2016 Jan; [PubMed PMID: 26392015]
Oshimi K. Clinical Features, Pathogenesis, and Treatment of Large Granular Lymphocyte Leukemias. Internal medicine (Tokyo, Japan). 2017:56(14):1759-1769. doi: 10.2169/internalmedicine.56.8881. Epub 2017 Jul 15 [PubMed PMID: 28717070]
Mandal A, Viswanathan C. Natural killer cells: In health and disease. Hematology/oncology and stem cell therapy. 2015 Jun:8(2):47-55. doi: 10.1016/j.hemonc.2014.11.006. Epub 2014 Dec 27 [PubMed PMID: 25571788]
Level 3 (low-level) evidencePaul S, Lal G. The Molecular Mechanism of Natural Killer Cells Function and Its Importance in Cancer Immunotherapy. Frontiers in immunology. 2017:8():1124. doi: 10.3389/fimmu.2017.01124. Epub 2017 Sep 13 [PubMed PMID: 28955340]
Krzewski K,Coligan JE, Human NK cell lytic granules and regulation of their exocytosis. Frontiers in immunology. 2012 [PubMed PMID: 23162553]
Kiessling R, Petranyi G, Kärre K, Jondal M, Tracey D, Wigzell H. Killer cells: a functional comparison between natural, immune T-cell and antibody-dependent in vitro systems. The Journal of experimental medicine. 1976 Apr 1:143(4):772-80 [PubMed PMID: 1082915]
Level 3 (low-level) evidenceHerberman RB, Bartram S, Haskill JS, Nunn M, Holden HT, West WH. Fc receptors on mouse effector cells mediating natural cytotoxicity against tumor cells. Journal of immunology (Baltimore, Md. : 1950). 1977 Jul:119(1):322-6 [PubMed PMID: 326962]
Level 3 (low-level) evidenceAbel AM, Yang C, Thakar MS, Malarkannan S. Natural Killer Cells: Development, Maturation, and Clinical Utilization. Frontiers in immunology. 2018:9():1869. doi: 10.3389/fimmu.2018.01869. Epub 2018 Aug 13 [PubMed PMID: 30150991]
Zhang Y,Huang B, The Development and Diversity of ILCs, NK Cells and Their Relevance in Health and Diseases. Advances in experimental medicine and biology. 2017; [PubMed PMID: 28921473]
Level 3 (low-level) evidenceStabile H, Fionda C, Gismondi A, Santoni A. Role of Distinct Natural Killer Cell Subsets in Anticancer Response. Frontiers in immunology. 2017:8():293. doi: 10.3389/fimmu.2017.00293. Epub 2017 Mar 16 [PubMed PMID: 28360915]
Lo Nigro C, Macagno M, Sangiolo D, Bertolaccini L, Aglietta M, Merlano MC. NK-mediated antibody-dependent cell-mediated cytotoxicity in solid tumors: biological evidence and clinical perspectives. Annals of translational medicine. 2019 Mar:7(5):105. doi: 10.21037/atm.2019.01.42. Epub [PubMed PMID: 31019955]
Level 3 (low-level) evidencePark JE, Kim SE, Keam B, Park HR, Kim S, Kim M, Kim TM, Doh J, Kim DW, Heo DS. Anti-tumor effects of NK cells and anti-PD-L1 antibody with antibody-dependent cellular cytotoxicity in PD-L1-positive cancer cell lines. Journal for immunotherapy of cancer. 2020 Aug:8(2):. doi: 10.1136/jitc-2020-000873. Epub [PubMed PMID: 32830112]
Sojka DK,Yang L,Yokoyama WM, Uterine Natural Killer Cells. Frontiers in immunology. 2019 [PubMed PMID: 31118936]
Fuchs A. ILC1s in Tissue Inflammation and Infection. Frontiers in immunology. 2016:7():104. doi: 10.3389/fimmu.2016.00104. Epub 2016 Mar 22 [PubMed PMID: 27047491]
Poli A, Michel T, Thérésine M, Andrès E, Hentges F, Zimmer J. CD56bright natural killer (NK) cells: an important NK cell subset. Immunology. 2009 Apr:126(4):458-65. doi: 10.1111/j.1365-2567.2008.03027.x. Epub [PubMed PMID: 19278419]
Tsuchiyama J, Yoshino T, Mori M, Kondoh E, Oka T, Akagi T, Hiraki A, Nakayama H, Shibuya A, Ma Y, Kawabata T, Okada S, Harada M. Characterization of a novel human natural killer-cell line (NK-YS) established from natural killer cell lymphoma/leukemia associated with Epstein-Barr virus infection. Blood. 1998 Aug 15:92(4):1374-83 [PubMed PMID: 9694726]
Level 3 (low-level) evidenceFrey T,Petty HR,McConnell HM, Electron microscopic study of natural killer cell-tumor cell conjugates. Proceedings of the National Academy of Sciences of the United States of America. 1982 Sep; [PubMed PMID: 6291036]
Level 3 (low-level) evidencePle C, Barrier M, Amniai L, Marquillies P, Bertout J, Tsicopoulos A, Walzer T, Lassalle P, Duez C. Natural killer cells accumulate in lung-draining lymph nodes and regulate airway eosinophilia in a murine model of asthma. Scandinavian journal of immunology. 2010 Aug:72(2):118-27. doi: 10.1111/j.1365-3083.2010.02419.x. Epub [PubMed PMID: 20618770]
Level 3 (low-level) evidenceYadav PK, Chen C, Liu Z. Potential role of NK cells in the pathogenesis of inflammatory bowel disease. Journal of biomedicine & biotechnology. 2011:2011():348530. doi: 10.1155/2011/348530. Epub 2011 Jun 1 [PubMed PMID: 21687547]
Kucuksezer UC, Aktas Cetin E, Esen F, Tahrali I, Akdeniz N, Gelmez MY, Deniz G. The Role of Natural Killer Cells in Autoimmune Diseases. Frontiers in immunology. 2021:12():622306. doi: 10.3389/fimmu.2021.622306. Epub 2021 Feb 25 [PubMed PMID: 33717125]
Orange JS, Natural killer cell deficiency. The Journal of allergy and clinical immunology. 2013 Sep; [PubMed PMID: 23993353]
Orange JS. How I Manage Natural Killer Cell Deficiency. Journal of clinical immunology. 2020 Jan:40(1):13-23. doi: 10.1007/s10875-019-00711-7. Epub 2019 Nov 22 [PubMed PMID: 31754930]
Ornstein BW, Hill EB, Geurs TL, French AR. Natural killer cell functional defects in pediatric patients with severe and recurrent herpesvirus infections. The Journal of infectious diseases. 2013 Feb 1:207(3):458-68. doi: 10.1093/infdis/jis701. Epub 2012 Nov 21 [PubMed PMID: 23175766]
Almerigogna F, Fassio F, Giudizi MG, Biagiotti R, Manuelli C, Chiappini E, Galli L, Romagnani S, De Martino M. Natural killer cell deficiencies in a consecutive series of children with herpetic encephalitis. International journal of immunopathology and pharmacology. 2011 Jan-Mar:24(1):231-8 [PubMed PMID: 21496407]
Zhang S,Liu W,Hu B,Wang P,Lv X,Chen S,Shao Z, Prognostic Significance of Tumor-Infiltrating Natural Killer Cells in Solid Tumors: A Systematic Review and Meta-Analysis. Frontiers in immunology. 2020; [PubMed PMID: 32714321]
Level 1 (high-level) evidenceWagner S, Wittekindt C, Reuschenbach M, Hennig B, Thevarajah M, Würdemann N, Prigge ES, von Knebel Doeberitz M, Dreyer T, Gattenlöhner S, Klussmann JP. CD56-positive lymphocyte infiltration in relation to human papillomavirus association and prognostic significance in oropharyngeal squamous cell carcinoma. International journal of cancer. 2016 May 1:138(9):2263-73. doi: 10.1002/ijc.29962. Epub 2016 Jan 8 [PubMed PMID: 26662627]
Ascierto ML, Idowu MO, Zhao Y, Khalak H, Payne KK, Wang XY, Dumur CI, Bedognetti D, Tomei S, Ascierto PA, Shanker A, Bear HD, Wang E, Marincola FM, De Maria A, Manjili MH. Molecular signatures mostly associated with NK cells are predictive of relapse free survival in breast cancer patients. Journal of translational medicine. 2013 Jun 12:11():145. doi: 10.1186/1479-5876-11-145. Epub 2013 Jun 12 [PubMed PMID: 23758773]
Liu S, Galat V, Galat Y, Lee YKA, Wainwright D, Wu J. NK cell-based cancer immunotherapy: from basic biology to clinical development. Journal of hematology & oncology. 2021 Jan 6:14(1):7. doi: 10.1186/s13045-020-01014-w. Epub 2021 Jan 6 [PubMed PMID: 33407739]
Liu E,Marin D,Banerjee P,Macapinlac HA,Thompson P,Basar R,Nassif Kerbauy L,Overman B,Thall P,Kaplan M,Nandivada V,Kaur I,Nunez Cortes A,Cao K,Daher M,Hosing C,Cohen EN,Kebriaei P,Mehta R,Neelapu S,Nieto Y,Wang M,Wierda W,Keating M,Champlin R,Shpall EJ,Rezvani K, Use of CAR-Transduced Natural Killer Cells in CD19-Positive Lymphoid Tumors. The New England journal of medicine. 2020 Feb 6; [PubMed PMID: 32023374]
Del Zotto G, Marcenaro E, Vacca P, Sivori S, Pende D, Della Chiesa M, Moretta F, Ingegnere T, Mingari MC, Moretta A, Moretta L. Markers and function of human NK cells in normal and pathological conditions. Cytometry. Part B, Clinical cytometry. 2017 Mar:92(2):100-114. doi: 10.1002/cyto.b.21508. Epub 2017 Feb 12 [PubMed PMID: 28054442]